Extrusion of cross-linked polyethylene and process of coating wire thereby



EXTRUSION F CROSS-LINKED POLYETHYLENE AND PROCESS OF COATING WIRETHEREBY No Drawing. Appl cati n August 30, 1957 Serial No. 681,175

Claims. or. 18-55) This invention relates to a process for extrudingcrosslinked polyethylene; more particularly. the invention relates to aprocess for extruding cross-linked polyethylene as insulation on wire.

During the past five years there have been a number of investigationsmade into the cross-linking of polyethylene by means of high energyelectron radiation. More recently, there has been a great deal ofinterest in the use of peroxides, such as di-a-cumyl peroxide, ascross-linking agents. Examples of patent applications assigned to thesame assignee as the present application which have been filed to covervarious aspects of crosslinked polyethylene are: Precopio and GilbertSerial No. 509,387, filed May 18, 1955 (Patent No. 2,888,424, issued May26, 1959), directed to chemically cross-linked polyethylene in which anumber of different filler materials' may be present; Precopio andGilbert Serial No. 509,388, filed May 18, 1955, directed toward a numberof different tertiary peroxides which may be used to achievecross-linking of polyethylene; Safford Se ial No. 550,834, filedDecember 5, 1955 (Patent No. 2,888,419, issued May 26, 1959) directedtoward a cross-linked polyethylene containing an organopolysiloxane;Safiord and Corrin Serial No. 554,627, filed December 22, 1955, directedtoward a chemically cross-linked polyethylene containing a basicmaterial as a stabilizer; and Cole Serial No. 561,937, filed January 27,1956. directed toward a cross-linked polyethylene which is subjected toa milling operatic-n prior to molding or extrusion.

The above-mention Precopio and Gilbert, Saiford, and

' Cole applications were directed to cross-linked polyethylenecompositions broadly rather than to compositions classifiable only aselectrical grade materials. Although States Patent 0 cross-linkedpolyethylene makes an excellent electrical insulation, serious problemshave been encountered in its fabrication. For example, insulation isextruded on wire which is then subjected toan elevated temperature tobring about cross-linking or cure. Since polyethylene which has not beencross-linked has a rather low melting point, there is a tendency for theextruded polyethylene to become fluid when the temperature is raised forcuring and the cure has not yet been effected. This problem can besolved by loading the polyethylene with a filler material to impart formstability at elevated temperatures; but fillers degrade the electricalproperties as by lowering the electrical resistance of the insulation.The maximum amount of filler material 'which can be present withoutseriously affecting the electrical properties is less than 20% by weightand this is insufficient to impart the necessary form stability duringthe cure of extruded insulation.

Oneof the objects of this invention is to provide a cross-linkedpolyethylene which, after extrusion, will retain its form during an opensteam vulcanization or cure.

Another object of the invention is to provide a crosslinked polyethylenesuitable for electrical insulation rCC which, after extrusion, remainsform stable when subjected to open steam curing.

Another object of the invention is to provide a process formanufacturing wire having an extruded insulating layer of cross-linkedpolyethylene.

Other objects of the invention will become apparent from a perusal ofthe following specification which, briefly stated, is directed to aprocess for extruding polyethylene which comprises mixing withpolyethylene up to 5 parts by weight of a guanidine per 100 parts ofpolyethylene and from 10% to 35% of the quantity of tertiary peroxidenecessary for optimum cross-linking, curing this mixture to elfect a gelstructure therein, mixing therewith the remaining 65% to tertiaryperoxide together with up to 20 parts by weight of filler per parts ofpolyethylene and up to 10 parts by weight of an organopolysiloxane per100 of polyethylene to produce an extrudable composition, extruding saidcomposition, and contacting the extruded product with steam at thecuring temperature of the polyethylene compound.

In the above brief statement of the invention, a-

guanidine, a filler, and an organopolysiloxane are optional components.While the present invention is not directed particularly toward theseoptional components, it is desirable that all be present since each oneconfers desirable properties on the final product. Safiord and CorrinSerial No. 554,627, cited above, pointed out that the presence ofguanidines inperoxide cross-linked polyethylene makes it less sensitiveto milling conditions that cause premature decomposition. It has beennoted that guanidines actually increasethe electrical resistance of thepolyethylene thus enhancing its electrical properties. Precopio andGilbert Serial No. 509,387, cited above, pointed out that fillers-suchas silica, carbon black, alumina, and calcium silicate, increase thetensile strength and percent elongation of peroxide cross-linkedpolyethylene. Sal'ford Serial No. 550,834, cited above, pointed outthata minor amout of an or'ganopolysiloxane in filled polyethyleneproduces a more extrudable composition having a smoother surface. Thus,the properties conferred by the presence of guanidines, fillers, andorganopolysiloxanes are all desirable when the crosslinked polyethyleneis used as electrical insulation. However, it must be emphasized thatelectrical requirements,

particularly in the case of fillers, limit the upper propor-' theinsulated wire is then slowly passed through a long steam chamber whichis maintained at a temperature sufiicient to effect a cure of theinsulation. The wire is then cooled and wound on a reel.

As an example of the operation of this invention, 100

pounds of polyethylene, 1 pound of diphenyl guanidine,

and 12 ounces of dicumyl peroxide are added to a warm (220 F.) Banburymixer which is then operated at a 220 F. temperature for four minutes.The temperature of the Banbury mixer is then increased to 325 F. for 15minutes and further mixing at this elevated temperature producessufiicient cross-linkage to impart a gel structure. The mixture is thenremoved from the Banbury mixer and allowed to cool. If desired, it maybe" stored for extended periods of time- As soon as it is desired toconvert the gel mixture described above into an extrusion mixture, 10pounds ''of finely divided calcium silicate and 5 pounds ofdimethylpolysiloxane are added along with 3 pounds of dicumyl peroxideand the composition is mixed on a Banbury mixer maintained at atemperature of 220 F. for 4 minutes. The extrusion mixture thus producedis then introduced to an extrusion apparatus having a tuber maintainedat approximately 235 F. As soon as the composition has been extruded onwire being drawn through the extrusion head, it is subjected to contactwith steam at 250 pounds pressure in order to bring about thecross-linking or cure of the polyethylene. As the temperature risesduring the final cure, the gel structure maintains the insulation in itsextruded form. The insulated wire is then cooled in a waterbath andwound on a reel.

It is to be emphasized that the temperatures set forth in the aboveexample are merely representative. The initial temperatures in theBanbury mixer and tuber are at a level to impart proper viscositywithout subjecting the mixture to a significant degree of cure. Thefinal temperature in the Banbury-during which time the initial gelstructure is producedand the temperature in the steam chamber aresufficiently high to bring about cross-linkage. Obviously, othertemperatures than those listed above would accomplish these results.

Alternatively, the mixtures produced in the above example could bepassed from the Banbury mixer directly to a mil for further treatment atlow temperature. in

the case of the gel mixture, milling produces a finely texturedstructure and in the case of the extrusion mixture, milling gives closecontrol of the consistency.

In the above example, a total of 3% pounds of dicumyl peroxide was used,the initial quantity added to bring about the gel structure being 20% ofthe total. More or less total peroxide may be used depending upon theproperties which it is desired to impart to the final insulated wire anddepending also on the quantities of guanidine, filler, andorganopolysiloxane used. Dicumyl peroxide may be used in a proportion asgreat as 10 parts by weight of peroxide to 100 of polyethylene. Inaddition, the percentage of the total peroxide initially added in orderto produce a gel structure may be varied from 10% of the total to asmuch as 35%. However, the 20% initial addition set forth in the aboveexample is an optimum figure.

In the above working example, 1 pound of diphenyl guanidine was usedalong with 10 pounds of calcium silicate and 5 pounds ofdimethylpolysiloxane, These proportions are optimum; however, it must beemphasized that the guanidine may be present in a proportion as great as5 parts by weight per 100 parts of polyethylene, the tiller may bepresent in a proportion as great as 20 parts by weight per 100 ofpolyethylene, and the organopolysiloxane in a proportion as great asparts by weight per 100 of polyethylene.

The preferred embodiment of the invention is set forth in the aboveworking example. Limits of the invention are clear from the Precopio andGilbert, Satford, and Cole applications previously cited herein. Forexample, Precopio and Gilbert Serial No. 509,387 points out that silica,carbon black, alumina, and calcium silicate are satisfactory fillermaterials and of these calcium silicate is the most useful in thepresent invention. Carbon black is the least desirable since it has themost adverse effect on the electrical resistance of insulation in whichit is incorporated. Safford and Corrin Serial No. 554,627 points outthat aryl substituted guanidines are most useful in conjunction withcross-linked polyethylene and that within this class diphenyl guanidineand di-tolylguanidine are most useful although guanidines within thescope of the following formula are satisfactory:

Where R's, which may or may not be different, are selected from thegroup consisting of hydrogen and a hydrogen radical, for example alkyl,e.g., methyl, ethyl, butyl, isobutyl, octyl, etc. radicals; cycloalkyle.g. cyclohexyl, cycloheptyl, etc. radicals; aryl e.g. phenyl, diphenyl,naphthyl, tolyl, xylyl, ethylphenyl, etc. radicals, and aralkyl, e.g.benzyl, phenylethyl, etc. These guanidines are also useful in'thcpresent invention.

While dicumyl peroxide is the preferred peroxide for achievingcross-linkage, other tertiary peroxides are satisfactory. As was pointedout in Precopio Serial No. 509,388, the tertiary peroxides of thepresent invention have the following formula:

where R and R, which may or may not be similar, are radicals selectedfrom the group consisting of:

where R R R and R comprise alkyl radicals such as methyl, ethyl, propyl,butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, octadecyl,etc. and isomers thereof; cycloalkyl radicals such as cyclopropyl,cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, etc.;alkylcycloalkyl radicals such as methylcyclobutyl, ethylcyclopentyl,tert-butyl-methylcyclohexyl, isopropylcyclohexyl, etc.; cycloalkyl alkylradicals such as cyclo propylmethyl, cyclopentylethyl, cyclohexylpropyl,etc.; aryl radicals such as phenyl, biphenyl, naphthyl, anthracyl, etc.;alkaryl radicals such as tolyl, xylyl, ethylphenyl, tert-butylphenyl,phenylbiphenyl, ethylnaphthyl, tert-butylnaphthyl, propylnaphthyl, etc.;aralkyl radicals such as benzyl, phenylethyl, naphthylpropyl, etc. Theunit is a radical wherein the tertiary carbon attached to the peroxideoxygen is contained within a hydrocarbon cyclic radical structure suchas cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, etc.,an example of which is diphenylcyclohexyl peroxide,

At least one member of the group consisting of R R R R R and R containsat least one aromatic group, Mixtures of these peroxides may also beused.

In addition, these peroxides may contain inorganic groups, such ashalogens, nitro groups, etc., for example, chlorophenyl, bromophenyl,nitrophenyl, etc.

Although di-a-cumyl peroxide is the most preferred peroxide, otherpreferred peroxides are tert butyl-u-cumyl peroxide CH3 CH3 CH: CH:

tert-butyltriphenyl methyl peroxide As pointed out in Safford Serial No.550,834, the organopolysiloxanes employed in the present inveniton areorganopolysiloxanes curable to the solid elastic state. The curableorganopolysiloxane or silicone compositions may be highly viscousmasses, or gummy elastic solids, depending on the state of condensation,the condensing agent employed, the starting organopolysiloxane used tomake the curable organopolysiloxanes, etc. Although these curableorganopolysiloxanes are well known, for

purposes of showing those skilled in the art the variousorganopolysiloxanes which may be employed in the present invention,attention is directed to the curable organopolysiloxanes disclosed andclaimed in Agens Patent No. 2,448,756; Sprung et a1. Patent No.2,448,556; Sprung Patent No. 2,484,595; Krieble et al. .Patent No.2,457,- 688; Hyde Patent No. 2,490,357; Marsden Patent No. 2,521,528;and Warrick Patent No. 2,541,137.

Curable organopolysiloxanes containing the same or differentsilicon-bonded organic substituents (alkyl, e.g. methyl, ethyl, propyl,butyl, octyl, etc.; alkenyl, e.g. vinyl, allyl, etc., cycloalkenyl, e.g.cyclohexenyl, etc.; aryl e.g. phenyl, tolyl, xylyl, naphthyl, etc.;aralkyl, e.g. benzyl, phenylethyl, etc.; halogenated aryl, e.g.chlorophenyl, dibromophenyl, fluorophenyl, etc.; cycloalkyl, e.g.cyclohexyl, etc.; alkynyl, e.g. ethynyl, etc.; both methyl and phenyl,etc. radicals) connected to silicon atoms by carbon-silicon linkages maybe employed without departing from the scope of the invention. 7

The particular curable organopolysiloxane used may be any one of thosedescribed in the foregoing patents and is generally obtained bycondensing a liquid organopolysiloxane containing an average of fromabout 1.9 to 2.1 silicon-bonded organic groups per silicon atom. Theusual condensing agents which may be employed and which arewell known inthe art may include, for instance, acid condensing agents, e.g. ferricchloride hexahydrate, phenyl phosphoryl chloride, and the like; alkalinecondensing agents, e.g. quaternary phosphonium hydroxides and alkoxides,solid quaternary ammonium hydroxides, potassium hydroxide, cesiumhydroxide, etc. These curable organopolysiloxanes generally comprisepolymeric iorganosiloxanes which may contain, for example, from to 2 molpercent copolymerized monorganosiloxane, for example, copolymerizedmonomethylsiloxane. Generally, the preferred starting organopolysiloxaneis one which contains 1.98 to 2.01 organic groups, for example methyl,per silicon atom where more than about 90% of the silicon atoms in thepolysiloxane contain 2 silicon-' bonded dialkyl groups.Dimethylpolysiloxane is a readily available organopolysiloxane and,therefore, is preferred for use in the present invention.

From the foregoing specification, it is obvious that 2 many variationsmay be made without departing from the spirit of the invention.Therefore, it is the intention that the invention be limited in scopeonly as may be necesstated by the scope of the-appended claims. 7

What we claim as new and desire to secure by Letters Patent of theUnited States is: p

1. A process for extruding polyethylene which comprises mixing withpolyethylene from 10% to 35% 0f the quantity necessary for optimumcross-linking of a peroxide in which each of the peroxide oxygens islinked directly. to a tertiary carbon atom whose remaining valences areattached to radicals selected from the group consisting of alkyl,cycloalkyl, alkyl cycloalkyl, aryl, and aralkyl, curing said mixture toeffect a gel structure therein, mixing therewith the remaining 65% to90% of said peroxide, extruding said composition, and contacting theextruded product with steam at the curing temperature to effect the curethereof.

2. A process as claimed in claim 1 wherein up to 5 parts by weight of aguanidine per 100 parts of polyethylene are present in the mixture atthe time the peroxide is initially added, up to 20 parts by weight offiller per 100 parts of polyethylene are added to the gel, and

up to 10 partsby weight of an organopolysiloxane per 100 parts ofpolyethylene are added to the gel.

3. A process as claimed in claim 2 wherein lpart by weight of aguanidine per 100 parts of polyethylene is present in the mixture.

4. A process as claimed in claim 2 wherein the organopolysiloxane ispresent to the extent of 5 parts by weight of the polyethylene.

5. A process as claimed in claim .1 wherein 20% of the peroxidenecessary for optimum cross-linking is present prior to formation of thegel and the remaining 80% is added in preparing the extrudablecomposition.

6. A process as claimed in claim 1 wherein the filler is calciumsilicate.

7. A process for making wire insulated, with crosslinked polyethylenewhich comprises mixing with poly ethylene from 10% to 35% by weight ofthe quantity necessary for optimum cross-linking of a peroxide in whicheach of the-peroxide oxygens is linked directly to a tertiary carbonatom whose remaining valences are at:

tached to radicals selected from the group consisting of alkyl,cycloalkyl, alkyl cycloalkyl, aryl, and aralkyl, cur. ing said structureto effect a gel structure therein, mixing.

therewith the remaining 65% to 90% of said peroxide,l

extruding said composition as insulation on a conductor, and contactingthe insulated conductor with steam to effect the cure of said extrudedinsulation.

8. A process as claimed in claim 7 wherein up to 5 i parts by weight ofa guanidine per 100 parts of polyethylene are present in the mixture atthe time the per-I oxide is initially added, up to 20 parts by weight ofa filler per 100 parts of polyethylene are added to the gel, and up to10 parts by weight of an organopolysiloxane per 100 parts ofpolyethylene are added to the gel.

9. A process as claimed in claim 7 wherein 20% of the peroxide necessaryfor optimum cross-linking is-present prior to the formation of the geland the remaining is added in preparing the extrudable composition. 10.A process as claimed in claim 8 wherein'l part by weight of a guanidineper parts of polyethylene.

is present'in the mixture;

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR EXTRUDING POLYETHYLENE WHICH COMPRISES MIXING WITHPOLYETHYLENE FROM 10% TO 35% OF THE QUANTITY NECESSARY FOR OPTIMUMCROSS-LINKING OF A PEROXIDE ILN WHICH EACH OF THE PEROXIDE OXYGENS ISLINKED DIRECTLY TO A TERTIARY CARBON ATOM WHOSE REMAINING VALENCES AREATTACHED TO RADICALS SELECTED FROM THE GROUP CONSISTING OF ALKYL,CYCLOALKYL, ALKYL CYCLOALKYL, ARYL, AND ARALYKLY, CURING SAID MIXTURE TOEFFECT A GEL STRUCTURE THEREIN, MIXING THEREWITH THE REMAINING 65% TO90% OF SAID PEROXIDE, EXTRUDING SAID COMPOSITION, AND CONTACTING THEEXTRUDED PRODUCT WITH STEAM AT THE CURING TEMPERATURE TO EFFECT THE CURETHEREOF.